1. Field of the Invention:
The present invention relates to threat signal detection systems for detecting the incidence of threat signals having at least one beam and, more particularly, to a threat detection system in which a controller condenses threat signals from harmonically related beams sorted from a group of detected pulses.
2. Description of the Prior Art:
As is known in the radar art, devices which employ active radar systems necessarily contain signal emitters whose emitted signals, commonly known as beams and so referred to herein, may be used to identify the device after the characteristics of the emitted signals, or beams, have been established. That is, devices which employ active radar systems contain at least one signal emitter which can be used to identify the device after the beam characteristics for the emitter are determined and have been associated with the particular device.
In the military sciences, devices which employ active radar systems include offensive and defensive weapons as well as their delivery vehicles. Although such weapons and their delivery vehicles are often used in a defensive posture, the presence of a weapon or a delivery vehicle will be considered to be a threat to the successful mission and/or survival of the opposing military force so that all such military devices employing active radar systems are, quite properly, designated as threats by the opposing force. Therefore, according to the general experience of the radar art as applied to use in the military sciences, it is well known in the prior art that military threats containing active radar systems may be detected in accordance with at least one beam which is known to be associated with the threat. Accordingly, the combination of one or more beams which emanate from a single threat have come to be known as a threat signal.
In the prior art, threat detection systems have detected threats by detecting beams which have characteristics within a predetermined range and comparing these detected beams to stored beams of similar characteristics which are known to be associated with a particular threat. In these prior art threat detection systems, the beams of the threat signals were detected by a receiver in response to the incidence of the beams on an antenna that is responsive to microwave energy. More particularly, a processor swept the receiver over predetermined frequency ranges and, when pulses were detected by the receiver, the processor extended the dwell of the receiver at that frequency to enable the receiver to collect a group of pulses which were stored in a buffer memory. The processor then sorted out pulses from this pulse group to form beams and collected harmonically related group had substantially the same radio frequency and was some multiple of a fundamental pulse repetition interval for the harmonically related beam group. Additional comparisons between the beams were then made to determine whether the beams were staggered with respect to each other. The beams thus formed were compared to stored beams which had a predetermined association with known threats so that, when beams were detected which matched the stored beams associated with a particular threat, the detected beams were recognized as detected threat signals whose further detection by the receiver could be predicted by a signal tracker.
These prior art threat detection systems operated on a premise that only beams which had substantially the same radio frequency and pulse repetition interval could be staggered with respect to each other as beams of a single threat signal. There were, however, situations in which this premise was inaccurate. For example, one may consider a threat signal which consists of three stagger levels, or beams, of equal PRI in which the first and third beams are staggered by one-half their pulse repetition intervals and the second beam is situated intermediate between the first and third beams. In this situation, the prior art threat signal detection system could sort out a single detected beam from the pulses of the first and third beams and would then sort out a second beam having the same radio frequency, but with a pulse repetition interval which was twice that of the first detected beam from the pulses of the second stagger level. Since the prior art threat detection system presumed that only beams having the same pulse repetition interval could be staggered with respect to each other as beams of a single threat signal, two threats would be detected when, in fact, only one threat existed. As a second example, consider the same threat signal as described above which, in this case, is sorted to form one detected beam having a PRI substantially equal to the interval distance between the first and third beams and three additional detected beams having PRI's substantially equal to three times the PRI of the first beam. Here again, the prior art threat detector would detect two threat signals because the prior art system presumed that only beams having the same PRI could be staggered with respect to each other as beams of a single threat signal. In both these examples, it is apparent that the threat detection system would never accurately detect the threat. Rather, the threat signals which were detected would represent non-existent threats which would serve to confuse the tracking system while, even more critically, the threat detection system would fail to recognize a physically existent threat.
The above-described situation in which a multiple of phantom threat signals are detected as a consequence of the sorting of a plurality of beams having different pulse repetition intervals became even more aggravated in those threat detection systems which would detect a beam of a threat signal despite the occasional absence of a pulse in the beam. Prior art threat detection systems were provided with this capability to enable them to detect beams in which occasional pulses were deliberately deleted from the transmission of the threat radar, or where the detection of an occasional pulse inadvertently failed. In these detection systems, pulses of the detected pulse group which were sorted to form a first beam of a given pulse repetition interval were assumed to have been undetected pulses when constructing beams having a different pulse repetition interval. The introduction of this capability of a threat detection system to ignore the occasional absence of a pulse when sorting beams from a pulse group magnifies the permutations of erroneously detected threat signals by orders of magnitude.
Subsequently, it has been realized that the errors of these prior art threat signal detection systems are a consequence of the failure of these systems to accurately detect the staggered nature of multiple stagger level threat signals. Further, it was realized that this inadequacy could be alleviated by considering all the detected beams which were harmonically related in pulse repetition interval to determine the stagger level of the threat signal since the erroneously detected threats arose as a consequence of the detected beams which comprise the threat signals being staggered by some multiple of a basic pulse repetition interval.